Knapsack-like problem with recipes and products -- help, please

[quinyu]

Given ingredients A, B, C, D, of which we have a, b, c, d amounts, we want to find the most profiting combination, knowing that for example recipe 1 uses 2 A and 3 B, and its product sells for 300 units, recipe 2 uses 2 B and 4 C and sells for 400 units, recipe 3 uses 1 A, 2 B and 1 D, and sells for 1000 units. We cannot manufacture fractional units, just integers. How would one go about this problem?

Thanks for all the suggestions in advance